The present invention concerns a soft magnetic material and a method for producing a soft magnetic material, in particular for use in solenoid valves.
Modern gasoline and diesel engines require increasingly high-performance solenoid injection valves in order to meet requirements for reducing fuel consumption and pollutant emissions. Fast-switching solenoid valves are needed in order to achieve these goals.
To implement these valves, soft magnetic materials with a high specific electrical resistance are utilized, for example, sintered FeSi, FeCr, or FeCo alloys, or soft magnetic composite materials comprising powdery iron and an organic binder.
Such soft magnetic composite materials often have a low mechanical stability and they are not sufficiently resistant to fuel and to high temperatures. In addition, in the case of the iron alloys which are obtained by sintering powdery materials, the only way to manufacture them with a specific electrical resistance of more than 1 xcexcxcexa9m is by way of alloying techniques.
One approach to increasing the specific electrical resistance of iron alloys is to coat a pure iron or iron-alloy powder with an electrically insulating layer before pressing, and then to sinter the compact into a mechanically stable shaped part.
The shaped parts obtained in this fashion have insufficient mechanical strength. In addition, during sintering, it is often not possible to retain the electrically insulating layer that was previously produced, so as to establish the desired high specific electrical resistance. Further, in the case of iron powders or iron-alloy powders, conventional simple pressing and sintering methods yield only limited densities, up to a maximum of 7.3 g/cm3, which are associated with a volume ratio of less than 92 vol % of the theoretical limit for the finished shaped parts.
German Patent Application No. DE 44 07 593 C1 describes a method for manufacturing high-density powder compacts. For this purpose, a conventional static pressing of pure iron powder in a die has superimposed on it a second process step in which the compact is acted upon during densification by brief pulses of current. This method is referred to in German Patent Application No. DE 44 07 593 as xe2x80x9cshock densification.xe2x80x9d
The present invention provides a soft magnetic material and a method for manufacturing the soft magnetic material. Embodiments of the present invention materials and method for manufacturing such materials having a highly dense and mechanically stable-shaped parts having extraordinarily good soft-magnetic properties.
In an embodiment of the present invention, the soft magnetic material that is manufactured has a high saturation polarization as well as very high specific electrical resistance values compared to iron materials and iron alloys produced by melting metallurgy. This high specific electrical resistance results, by way of the consequent decrease in eddy-current losses, in much-improved switching dynamics, for example, in solenoid valves.
In embodiments of the present invention, the soft magnetic materials that are obtained are moreover highly dimensionally stable, but can nevertheless also easily be mechanically reworked if necessary.
In addition, certain embodiments of the present invention can have a very high material density of more than 7.4 g/cm3, in particular more than 7.6 g/cm3.
Because the individual powder particles of the metallic powdery initial component are welded to one another by way of their high-resistance surface layers, the soft magnetic material that is obtained according to the present invention is mechanically very stable, temperature-resistant, and fuel-resistant.
In embodiments of the present invention, the metallic powdery initial component can be inexpensively obtainable commercially, and can easily be prepared for the method of the present invention.
In a further embodiment of the present invention, powders whose average particle size is more than 50 xcexcm, for example, between 100 xcexcm and 500 xcexcm, are used as initial powders.
In a further embodiment of the present invention, the average particle size of the powder particles of the metallic initial component is considerably greater than the thickness of the high-resistance surface layer. The highest possible proportion of powdery, e.g., ferritic or ferromagnetic initial component, relative to the high-resistance surface layer, can thereby be obtained in the soft magnetic material.
In the method according to the present invention, a sintering step or sintering operation that is otherwise usual in powder metallurgy can be omitted. Instead, the insulating layers on the surfaces of the individual powder particles are welded to one another by shock densification rather than being destroyed, which is unavoidable in the case of sintering.
In a further embodiment, particularly suitable as the metallic powdery initial component is a pure iron powder or iron-alloy powder that is then equipped superficially with a high-resistance layer, for example, an oxide layer made of Fe3O4 In order to establish a desired specific electrical resistance, this high-resistance surface layer may have a thickness of 1 xcexcm to 10 xcexcm.
Because the high-resistance surface layer that is present or produced at the surface of the initial powder particles is largely retained after the coated powder particles are densified into the soft magnetic material, and because the high-resistance surface layers between the individual powder particles are welded to one another by shock densification, a specific electrical resistance of more than 1 xcexcxcexa9m, in particular more than 2 xcexcxcexa9m, in the material is obtained.
In embodiments of the present invention, in the shaping of the powder particles of the metallic powdery initial component equipped with the high-resistance surface layer, the latter is placed in conventional fashion into a die, and densified by uniaxial pressing at a pressure of 200 MPa to 800 MPa. This shaping step can have superimposed on it the actual shock densification of the powder particles equipped with the high-resistance surface layer. For that purpose, pressing and shock densification of the compacts are performed in the die in one process step.